Precision thin film resistors are in general use in Si-based microelectronics integrated circuit chips. These resistors are frequently fabricated from polysilicon layers deposited on the chip. The resistor material is generally high in electrical resistivity, and the total resistance is controlled by the film thickness and the width and length of the rectangular film segment that is used. The resistance is given by R=rho*l/A, where rho is the electrical resistivity, l is the length (direction parallel to the current flow) of the rectangle, and A is the cross-sectional area (thickness times width). The resistivity of Si resistors can be tailored by implantation of dopants, which increase the conductivity.
However, silicon on insulator (SOI) wafers, low dielectric constant (e.g., low k) Back-End-Of-Line (BEOL) films, groundrule scaling, and thinner films are resulting in increased challenges in preventing overheating of semiconductors. Resistors produce heat when current flows through them and the effects of this heat on the performance of the resistors, and the nearby transistors are particularly problematic. Particularly, problems arise because resistors are being sandwiched between two insulative layers: a buried oxide (BOX) layer and BEOL films. In addition, the thickness of Si or polysilicon resistors is being reduced generationally, and resistor dimensions are limited by internal heating.
Simulations have shown that current Si resistors can heat up by as much as 10° C., which results in a 5% variation in the resistivity of the Si resistor. The generated heat can permanently alter the value of the resistance by changing the grain size of the polysilicon, by burning out portions (or all) of the film and by redistributing the dopant atoms. Due to these effects, it's necessary to limit the amount of current that the resistor can tolerate. In addition to effects on the resistor itself, the generated heat may be conducted into the metal lines that are connected to the resistor and also into metal lines that may be located immediately above the resistor. Heating of the attached and nearby metal structures increases the susceptibility of the metal to electromigration, a process that produces holes in the metallization in response to current flow and can eventually alter the electrical properties of the wire. Consequently, limiting the current through the resistor protects both the resistor stability and the integrity of the nearby metallization.
However, limiting the current through a resistor is at odds with the continued drive toward circuit miniaturization and the trend toward progressively greater current densities for high-performance circuits. That is, the heating constraint on resistor current is contending with circuit miniaturization and with circuit power requirements.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.